Protein engineering is becoming a widely used tool in many areas of protein biochemistry. One engineering method is controlled protein ligation. Native chemical protein ligation relies on efficient preparation of synthetic peptide esters, which can be technically difficult to prepare for many proteins. Recombinant technologies can be used to generate protein-protein fusions, joining the C-terminus of one protein with the N-terminus of another protein. Intein-based protein ligation systems can also be used to join proteins. A prerequisite for this intein-mediated ligation method is that the target protein is expressed as a correctly folded fusion with the intein, which is often challenging. The difficulties of conventional native and recombinant ligation technologies significantly limit the application of protein ligation.
The transpeptidation reaction catalyzed by sortases has emerged as a general method for derivatizing proteins with various types of modifications. For conventional sortase modifications, target proteins are engineered to contain a sortase recognition motif (LPXT (SEQ ID NO: 144)) near their C-termini. When incubated with synthetic peptides containing one or more N-terminal glycine residues and a recombinant sortase, these artificial sortase substrates undergo a transacylation reaction resulting in the exchange of residues C-terminal to the threonine residue with the synthetic oligoglycine peptide, resulting in the protein C-terminus being ligated to the N-terminus of the synthetic peptide.